Over the last 30,000 years, the human brain has decreased about 10% in size. This might seem like proof that we're not as smart as our ancient ancestors, but the truth is a little more complicated than that. Our brains aren't just getting smaller - they're also getting more efficient.

The research is simple enough. Based on skulls found in Europe, Asia, and the Middle East, the human brain was about 1,500 cubic centimeters 30,000 years ago, and now it's down to about 1,359 cubic centimeters. That means that, as a species, humans now only need 90% the brain size that we did 30,000 years ago. That's an incredibly rapid change - 30 millennia is practically no time at all in evolutionary terms - and one for which anthropologists have plenty of explanations.

First up, there's the physiological answer. It's easy to equate brain size with cognitive ability, but it's only part of the story. Brain size is also determined to a great extent by the overall size of an organism - after all, a sperm whale's brain is considerably larger than that of a human's, but that doesn't mean whales are smarter than humans. It just means whales are generally bigger than us, and that might be the case with our ancestors as well.

For instance, we know Neanderthals had bigger brains than us, but all available evidence indicates they were not the intellectual equals of our ancestors. However, they were more physically imposing than our ancestors, and our ancient counterparts were in turn bigger and stronger than we are today. It isn't just that bigger humans will need bigger brains as a matter of course - the increased size actually demands more gray matter to control the body properly. A population or species that's just generally more physically powerful than we are will need bigger brains just to handle basic tasks.

The next possibility is the only one that does suggest our shrinking brains are a sign of decreased intelligence. University of Missouri psychology professor David Geary suggests brain size decreased because the rise in population density took a lot of pressure off of individuals to solve problems, because now they could rely on their friends and relatives. The end result is simple enough, says Geary:

"As complex societies emerged, the brain became smaller because people did not have to be as smart to stay alive."

This does make a certain degree of intuitive sense, particularly to anyone who has ever realized they wouldn't last ten minutes without all the conveniences of modern living. After all, the price of stupidity was far greater at a time when people could not rely on the help of others, as any dumb mistakes could mean instant death. But it does seem like a rather extreme jump to then say that the removal of this pressure has actually made our species dumber - after all, haven't the last 30,000 years created new challenges that tax our brains in ways our ancestors could not even have imagined?

In a sense, that's what Duke University anthropologist Brian Hare suggests. He says our brains deal with information far more efficiently than our ancestors' did, and we have developed a more sophisticated form of intelligence that requires less raw brainpower.There's an obvious parallel for this elsewhere in the animal world, although you may not like hearing it: domesticated animals.

Yes, just like we have smaller, sleeker brains than our bigger ancestors, domesticated animals tend to be smaller and smarter than their wild counterparts. Dogs and wolves are a good example - wolves are stronger, have bigger brains, and the pressures of the wild arguably means there's a lower limit for wolf intelligence, as anything dumber than that won't be able to survive for long.

But while household dogs can occasionally get away with being really dumb, overall dogs are much smarter than wolves, even though they have smaller brains. After all, dogs are able to understand basic commands from humans, a remarkable feat of social learning of which wolves are pretty much entirely incapable.

We humans are obviously capable of even more complex learning than dogs, but the analogy is still a good way of understanding why our smaller brains don't actually mean we're getting any dumber. Of course, it's also pretty good proof that we've successfully domesticated ourselves, but I suppose we'll just have to live with that.